The present invention relates to the steering system of vehicles, and more particularly to the four-wheel steering system which prevents the instability of car body, so-called yawing, and the vibration of the vehicles during steering at high - speed running, by steering both front wheels and rear wheels simultaneously and properly.
The common steering apparatus changes the direction of front wheels by gearing the tie rod to the left or right relating to the rotating direction of the gear installed at the end of a steering shaft. However, the technical improvement of this kind of steering apparatus comprises the power steering which makes the smooth operation of steering wheel, or keep stability of car body at steering.
By the way, the above common steering apparatus has a problem that a car body becomes inclined suddenly and unstable while steering at high speed running, led away by a car body while running curved roads and thus becomes a primary factor for a side abrasion of a tire.
The above-mentioned is explained in detail as follows.
The unstability of the center of a car body may be softened to a certain extent by the operation of rear wheels-differential gear while steering at normal speed running, but while steering at high speed running, the center of a car body is suddenly leaning towards the inner side of steering, then the differential response becomes late and causes yawing of a car body and a severe abrasion of the tire. This phenomenon shows up more significantly in case of front driving type vehicle which have no differential apparatus at rear wheels.
On the other hand, there has been suggested four-wheel steering system which devices the solution of all drawbacks said above by steering rear wheels in proportion to the steering condition of front wheels. One conventional example of the four-wheel steering system is disclosed in Japanese patent provisionally published application No. 58-20565. In this system, the steering input given when a driver operates a steering wheel is transferred to front wheels and rear wheels simultaneously, then rear wheels are steered at the same value as the steering angle of front wheels by rear wheels steering apparatus.
This method has an advantage that the structure is comparatively simple, but does not get the satisfactable steering stability of a vehicle because front wheels and rear wheels can not be steered respectively due to the condition that the transmission characteristics between a steering input and front wheels steering angle, and the transmission characteristics between a steering input and rear wheels steering angle are set up uniformly with non-variableness.
There is another method of four-wheel steering system.
It is a method that an electronic control unit controls front wheels and rear wheels respectively to be steered with the optimum condition according to the relation between the speed of a vehicle and a steering input.
The examples of the four-wheel steering system with this electronic control method are disclosed in U.S. Pat. Nos. 4,687,214, 4,705,131 and 4,715,466. Most of them featuring the method that the most optimum front and rear wheels steering angle ratio corresponding to the speed of a vehicle (at high speed running of a vehicle) are memorized in the control unit as data and on the other hand the steering actuator operated by the oil pressure is independently set up to front wheels and rear wheels respectively and thus the control unit controls the oil pressure supplied to front and rear wheels actuator respectively according to the signal inputted through the speed sensor of a vehicle while steering at high speed running.
By the way, the said electronic control method has a problem that it is hardly to get the correct and optimum data for steering of front and rear wheels at high speed running because we should take into consideration many external factors, for examples, the speed of a vehicle, the road condition, weather, the driver's style of driving etc, and, moreover, has problems such as decrease of assembly productivity, increase of production cost and difficulty of repairing and maintenance because of the complexity in circuit which steers and controls front wheels and rear wheels respectively.